This invention relates generally to a mixer circuit which receives an input microwave signal and converts it to an intermediate frequency (IF) signal of a lower frequency. More particularly, the present invention relates to a novel mixer circuit which is capable of receiving two local oscillation signals having different frequencies.
In a conventional single mixer circuit containing one mixer diode, one of the terminals of the mixer diode is connected to the end of a main line which receives an input microwave signal, and the other end of the mixer diode is connected to a line for taking out an intermediate signal. In this case, a local oscillation signal is supplied to the main line through a bandpass filter. If two kinds of local oscillation signals are to be alternatively supplied to the single mixer circuit of the kind described above in order to expand a reception frequency band, it is necessary to employ a circuit construction in which two local oscillation signal supply lines are aligned in parallel with each other on one side of the main line or two local oscillation signal supply lines are disposed on both sides of the main line.
However, in such circuit construction where the two local oscillation signal supply lines are coupled to the main line together, more conversion loss is derived and the conversion characteristic deteriorates. Further, a larger circuit space is necessary. For these reasons, the mixer circuit having such circuit construction is not practical.